The source of cosmic rays, fast-moving particles that bombard Earth from space, has long been a mystery. Now scientists have found evidence implicating the remnants of dead stars.

The work suggests that the protons that make up the bulk of the rays emerge from remains of supernovae, the explosions that mark stellar deaths.

Cosmic rays form a sea of energetic particles in the galaxy. They were first detected in 1912, and a link to supernovae was suggested in 1933.

Electrons, another component of cosmic rays, were already known to be accelerated to the proper energies by supernova remnants. Scientists knew that if protons are also sped up by shockwaves of such remnants to become cosmic rays, they could give rise to gamma rays with a particular distribution of energies. In the new work, scientists found that gamma rays from a particular supernova remnant showed the telltale energy distribution.

The work is reported in the April 25 issue of the journal Nature. The results are strong evidence that the cosmic rays come from supernovae, Felix Aharonian of the Max Planck Institute for Nuclear Physics in Germany, wrote in an accompanying commentary.